Effect Of Environmental Factors On The Aging Mechanism Of Calf Leather

The long history and splendid culture provide us with rich cultural heritage,and leather cultural relics are one of the most important parts of it.However,as a collagen-based material,leather is susceptible to the temperature,humidity,ultraviolet irradiation and microorganisms,resulting in the deterioration of structure and properties during long-term burial.Apart from the environmental factors,the aging process of leather varies with the type of tanning agents because the composition,structure and tanning mechanisms of the leather tanned with different tanning agents are different.The determination of aging mechanism is the theoretical basis for the implementation of corresponding protection.However,the theoretical knowledge is too less to guide the identification and protection of leather cultural relics.In this paper,accelerate aging method is employed to systematically study the effect of the tanning agent types,UV irradiation,temperature and relative humidity（RH）,and fungi on the aging mechanism of different tanned leathers,in order to provide theoretical guidance for the conservation and restoration of leather artifacts.1.The thermal stability and thermal degradation behaviors of leather is related to its structure,as well as the tanning agent type.In this paper,the hydrothermal stability,evolved products and kinetics of thermal degradation of untanned hide and leather tanned with formaldehyde,glutaraldehyde,ME and TA were investigated.The differences in thermal stability and thermal degradation behavior of leather tanned with different tanning agents are analyzed.After tanning,the hydrothermal shrinkage temperature（T_s）of hides is improved.The T_s of glutaraldehyde-tanned leather is the highest among these four leathers,followed by ME tanned leather and the T_s of TA-tanned leather is the lowest.The main evolved composition during thermal degradation of the samples are CO₂,NH₃,pyrrole（C₄H₅N）,isocyanic acid（HNCO）and H₂O.However,the evolved peak temperature of products during thermal degradation of untanned hide and formaldehyde-tanned leather is higher than that of glutaraldehyde,ME and TA-tanned leathers.In comparison with untanned hide and leather tanned with formaldehyde,glutaraldehyde and ME,the initial evolved temperature of the products during thermal degradation of TA-tanned leather is low,indicating poor thermal degradation stability.Thus,TA-tanned leather is vulnerable to the environmental factors.2.Ultraviolet light（UV）is the main factor leading to the photoaging of leather.The leather tanned with formaldehyde,glutaraldehyde,ME and TA were irradiated with UV light for 0,12,24,48,72 and 96 h to investigate the UV aging resistance of leather tanned with different tanning agents.After UV irradiation,the color changes of ME and TA-tanned leathers are greater than that of aldehyde tanned leather,which should be due to the fact that ME and TA are easily oxidized to quinones by UV light,thereby resulting in obvious color changes of ME and TA-tanned leather.UV irradiation could cause photodegradation or photooxidation of some amino acids in leather such as cysteine,methionine,proline,hydroxyproline,phenylalanine,etc.,leading to the changes of the evolution yield of SO₂,pyrrole and toluene.The changes of the tensile properties of leather caused by UV irradiation are the results of the photodegradation,photooxidation and photo crosslinking of the tanning agent and collagen fibers in leather.3.Temperature and RH are key factors affecting the long-term storage of leather artifacts.The newly prepared leather tanned with formaldehyde,glutaraldehyde,ME and TA were artificially aged at 80°C and 60%RH for 0,2,4,8,16,24 and 32 days.Then the changes in the microscopic morphology,structure,and thermal degradation behaviors while being aged were investigated.After aging,the gaps between fiber bundles of formaldehyde-tanned leather are increased,which are decreased for glutaraldehyde,ME and TA-tanned leathers.In addition,the fiber bundles of ME and TA-tanned leathers become thinner.The fiber cohesion turns to be lost and the orientation of the collagen fibers is gradually destroyed.The T_s of the all the leathers is decreased,indicating that detanning happened in the leather by aging.After aging at80°C,60%RH for 32 days,the yield of nitrogen-containing products（NH₃,HNCO and C₄H₅N）during thermal degradation of formaldehyde-tanned leather are increased,while no obvious changes were found in the thermal degradation products of glutaraldehyde-tanned leather.For ME and TA-tanned leathers,the CO₂ yields during the thermal degradation are significantly increased,accompanied by the decreasing yield of nitrogen-containing products.The yield changes of evolved products indicated different structural changes for aldehyde-tanned leather and vegetable-tanned leather by aging.4.Penicillium and aspergillus niger are the two most common fungi that cause microbial damage for leather.In order to simulate the microbial aging process of leather,TA and ME tanned leathers are cultured in the environment with penicillium and aspergillus niger for 0,15,30,60 and 90 days.The changes in the morphology of the grain layer,structure and tensile properties of TA and ME tanned leathers were investigated.After being aged with penicillium and aspergillus niger,the pore structure of grain layer of leather is broken.Both the TA and the collagen in TA-tanned leather were destroyed by aging.Therefore,the tensile properties of TA-tanned leather are decreased.However,the structure broken and deterioration of tensile property for ME-tanned leather caused by penicillium and aspergillus niger is less than that of TA-tanned leather,which suggested that the aging resistance of the ME-tanned leather is better than that of the TA-tanned one.